Pettitte, 34, began to experience tightness and spasms following a regular workout involving squats on Monday. Pettitte felt fine leaving Legends Field but said his back stiffened at home, then again on Tuesday morning.
"I'm pretty locked up," Pettitte said.
Torre said that that the left-hander would probably not be available to pitch on Friday, the next day his turn would have come around the rotation.
"I think it's safe to assume that," Torre said. "During the season, you may think differently."
Pettitte had originally been scheduled to throw on the side Tuesday, and Torre said it was doubtful that Pettitte would be able to take part on Wednesday. Pettitte had been slated for a Minor League game on Friday, with right-hander Darrell Rasner tabbed to start for the big-league squad against the Pirates.
"I want to stay on schedule," Pettitte said, "but I also realize that it's early."
Torre said he was not aware of any further tests, as the general consensus seems to conclude that the back spasms are not indicative of anything larger.
Pettitte said he had received treatment on Tuesday and was feeling looser.
"We're proceeding very cautiously here, even though it doesn't appear to be anything more than what it is," Torre said.
Pettitte -- back with the Yankees after three years in the National League with the Astros -- has not allowed a run in 10 spring innings this season, limiting opponents to five hits while walking two and striking out seven.
Along with Chien-Ming Wang, Pettitte is one of the Yankees' leading candidates to pitch on Opening Day, April 2, against the Rays. Torre said that missing one start is not likely to jeopardize Pettitte's standing for the regular season.
"Obviously, if they don't go away, it's going to mean something," Torre said. "You're getting close to the finish line. I don't anticipate it being a major problem."
Bryan Hoch is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.